class Solution {
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {1, -1, 0, 0};
public:
    int maxDistance(vector<vector<int>>& grid) {
        int m = grid.size(), n = grid[0].size();
        vector<vector<int>> dist(m, vector<int>(n, -1));
        queue<pair<int, int>> q;
        for(int i = 0; i < m; i++) // 处理边界的1
        {
            for(int j = 0; j < n; j++)
            {
                if(grid[i][j])
                {
                    dist[i][j] = 0;
                    q.push({i, j});
                }
            }
        }
 
        int ret = -1; // 全是海洋返回-1
        while(!q.empty())
        {
            auto[a, b] = q.front();
            q.pop();
            for(int i = 0; i < 4; ++i)
            {
                int x = a + dx[i], y = b + dy[i];
                if(x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;
                    ret = max(ret, dist[x][y]);
                    q.push({x, y});
                }
            }
        }
        return ret;
    }
};